1. Field of the Invention
The present invention relates to a scanning optical apparatus used for a laser beam printer apparatus capable of appropriately performing temperature compensation for a plastic lens applied to the optical system of the scanning optical apparatus and, more particularly, suppressing the curvature of the scanning line in an oblique incident optical system and reducing pitch or density variations in an image.
2. Related Background Art
Conventionally, a variety of multi-beam scanning optical apparatuses in which a plurality of light beams are sent onto a common scanning optical system to realize multiple colors have been proposed. In a multi-beam scanning optical apparatus of this type, when a plurality of light beams are to be independently irradiated on a predetermined target scanning surface, the light beams must be deflected/reflected by an optical deflector comprising, e.g., a polygon mirror and then separated. Light sources in the same wavelength band must be spatially separated. When light beams are obliquely incident on the deflection surface of the optical deflector within a sub scanning section, desired spatial separation is enabled.
However, in an enlargement optical system having a compact optical arrangement, the optical path length for spatial separation is short, and therefore, the oblique incident angle with respect to the deflection surface increases, resulting in a curvature of the scanning line on the target scanning surface.
To solve this problem, the present inventors have proposed, in Japanese Patent Application No. 8-174332, a technique using a toric lens whose generating-line is three-dimensionally curved and whose optical axis is decentered within a plane perpendicular to the on-axis incident light beam.
This toric lens has complex surface shapes. A lens having such complex surface shapes is manufactured by molding using a plastic material. However, the optical nature of a plastic material generally largely varies depending on the environment as compared to a glass material. For example, a change in refractive index of a plastic material due to a change in temperature is an order of magnitude or more larger than that of a glass material.
When the toric lens is manufactured without considering such an environmental variation, the focal point position shifts at elevated temperatures. For this reason, a curvature of the scanning line occurs on the image plane at elevated temperatures.